1. Field of the Invention
The present invention relates to a method of and apparatus for forming a thin film, especially, an antistatic or antireflection film on an outer surface of a display screen of a cathode ray tube.
2. Description of the Related Art
Normally, the outer surface of a display screen of a cathode ray tube has a smooth mirror-like surface, and its surface resistance is high. For this reason, external light is reflected on the outer surface, and therefore an image displayed on an inner surface of the display screen is dull or a static charge is undesirably stored during an operation of the cathode ray tube. In order to prevent such inconveniences, it is well known to form a thin film, especially, an antistatic or antireflection film on the outer surface of a display screen of a cathode ray tube. For this purpose, several manufacturing methods have been proposed.
For example, Japanese Patent Disclosure No. 61-118932 describes a method of coating an alcoholic solution of Si(OR).sub.4 on the outer surface of a display screen of a cathode ray tube by spray coating, drying and baking the coated solution, thereby forming an SiO.sub.2 film having antistatic and antidazzling effects.
This method aims at obtaining an antistatic effect of an SiO.sub.2 film by forming an SiO.sub.2 film having fine projections on the outer surface of a display screen, which diffuse external light, thereby obtaining an antidazzling effect. However, the above method is not suitable for forming a smooth thin film without projections such as a thin film having only an antistatic effect or an antireflection film which utilizes interference of external light, especially when a solution to be coated is volatile. The reason for this is as follows.
FIGS. 1A and 1B schematically show a formation process of an SiO.sub.2 film performed by a spray coating method. In FIG. 1A, droplets 2 of an Si(OR).sub.4 alcoholic solution are adhered to an outer surface of a display screen of a cathode ray tube by the spray coating method. When droplets 2 fly in the air in and apparatus for arrow a large amount of alcohol as a solvent is evaporated and lost. Therefore, hydrolysis and condensation of Si(OR).sub.4 progress and the viscosity of the droplets is increased. For this reason, the droplets are adhered on the outer surface of the display screen and become semi-spherical droplets 3. Since semispherical droplets 3 adhered on the outer surface of the display screen continue to be dried, hydrolysis and condensation of Si(OR).sub.4 progress. Therefore, when subsequent droplets are adhered on droplets 3, they do not mix with one another, resultantly forming the projections 4. As a result, SiO.sub.2 film 5 having fine projections on its surface as shown in FIG. 1B is formed. This phenomenon occurs whenever a volatile solvent such as an alcohol is used.
In order to form a film having a surface without projections by the spray coating method, droplets must be adhered to the display screen before the solvent in the previously adhered droplets has evaporated. For this purpose, it is conceivable to increase a spray amount of the Si(OR).sub.4 alcoholic solution per unit time. In addition, an airless-spraying machine may be used in place of an air-spraying machine and the distance between the airless-spraying machine and the display screen may be shortened to minimize the evaporation of the solvent while the droplets fly in the air.
By way of these methods, including the spray coating method, a smooth thin film without projections can be formed. However, the process of forming a uniform thin film having a desired thickness on the outer surface of a display screen of a cathode ray tube remains a difficult task, especially in a large cathode ray tube.
A dipping method is also known to those skilled in the art as a method of forming a smooth thin film without projections. In this method, after an outer surface of a display screen of a cathode ray tube is dipped in a film formation material solution, the display screen is raised upright, and then the solution is caused to dry. With this method, a thin film having a smooth surface can be formed. However, since the solution flows downward while it is being dried, a film thickness of a lower portion is increased. In a display screen having a large area, this difference in the film thickness is increased, and therefore a thin film having a uniform thickness is hard to form.
In addition to the above dipping method, a rotating method is also known as a method of forming a smooth thin film on a substrate. In this method, a film formation material solution is applied on the entire surface of a substrate and then the substrate is rotated at a high speed, thereby forming a uniform solution film. This method is effective when a substrate surface is flat and has a relatively small area. However, when a solution containing a volatile solvent is to be applied on a substrate having a convex surface with a relatively large area such as a display screen of a cathode ray tube, it is difficult to directly apply the method.
The reason for this is as follows. That is, in this method, after the solution film is formed by applying the film formation material solution on the outer surface of the display screen of the cathode ray tube to form a solution film, the cathode ray tube is rotated about its tube axis at a high speed (about 130 to 200 rpm) in order to obtain a uniform thickness of the solution film. Then, the excess solution film applied near the center of the display screen is moved to a periphery of the display screen by a centrifugal force and then dried and solidified at the periphery by an air flow generated by rotation, thereby forming annular projections. Since an air flow generated by the rotation of a central portion is weak and hence a solution at this portion is slowly dried, not only the excess portion of the solution but also a solution required for obtaining a desired film thickness are caused to flow to the periphery of the display screen. As a result, the difference in a film thickness is further increased.
The solution moved from the central portion to the periphery of the display screen causes an inferior, nonuniform thickness in the film at the corners of the display screen because a surface of the display screen of the cathode ray tube is convex and square. That is, in FIG. 2, when the cathode ray tube rotates in a direction indicated by arrow a, the solution applied on a portion near the central portion flows in directions indicated by arrows b. Although the solution mostly spreads outwardly, some of the solution stays at an edge portion of the display screen and flows in a direction indicated by arrows c. A solution which flows to the hatched portions then flows in the directions indicated by arrows d to the corners of the display screen because the display screen is convex and square. Since the speed of air flow generated at the corners is high, the solution film is dried faster in the corner areas than in the central portion, and a solution from the central portion further flows onto the dried film in the corner areas and is dried thereon, thereby forming a thick film.